1. Field of the Invention
The present invention relates to iron oxide pellets which are to be reduced in a rotary hearth furnace or the like and to a method of producing the iron oxide pellets. The present invention also relates to reduced iron pellets obtained through reduction of the iron oxide pellets and to a method of producing the reduced iron pellets.
2. Description of the Related Art
The Midrex method is a well-known method of producing reduced iron. In the Midrex method, a reducing gas produced from natural gas is fed through a tuyere into a shaft furnace and allowed to rise therein for reduction of iron ore or iron oxide pellets charged therein, to thereby produce reduced iron. However, since the method requires a supply, as a fuel, of a large amount of high-cost natural gas, the location of a plant utilizing the Midrex method is limited to a region producing natural gas.
In recent years, a certain type of methods for producing reduced iron has become of interest, in which instead of natural gas relatively inexpensive coal can be used as a reducing agent. An example of such a method is disclosed in U.S. Pat. No. 3,443,931. In this prior art technique, a mixture of a powder of iron ore and a carbonaceous material is pelletized, then reduced in a high-temperature atmosphere to thereby produce reduced iron. This method has the following advantages among others: Coal can be used as a reducing agent; a powder of iron ore can be used directly; reduction is performed at a high rate; and the carbon content of a product can be regulated.
However, since carbonaceous material has substantially no effects of binding pellet granules together, the strength of carbonaceous-material-containing iron oxide pellets is low as compared with that of pellets containing no carbonaceous material. If the strength of green pellets before drying is low, the pellets are crushed and pulverized in the handling during the drying process, resulting in a low yield of iron oxide pellets. Also, if the strength of iron oxide pellets after drying is low, the pellets are crushed and pulverized when fed into a reducing furnace, resulting in a low yield of reduced iron. The pulverization occurring during feeding of the pellets also leads to lowered quality of reduced iron pellets.
Japanese Patent Publication (kokoku) No. 52-26487 discloses a prior art technique directed to improvement of the strength of reduced iron pellets in a reducing process and that of dried iron oxide pellets. In this prior art technique, bentonite (a coagulating agent) is added in an amount of 1 mass % or more to a combination of fine powder of ore material and a carbonaceous reducing agent, and the resultant mixture is kneaded with conditioning water prepared by dissolving a dispersing agent (0.3 mass % or less) in an organic binder such as starch, and granulated while an adequate amount of water is sprayed thereon, to thereby obtain pellets.
This prior art technique enables improvement of the strength of pellets, but has disadvantages as follows:
A first disadvantage will be described. Since bentonite serving as a coagulating agent has a property of swelling to a great extent, a large amount of water must be added during the pelletization step by use of a pelletizer. Addition of water leads to softening and easy deformation of pellets. The deformation hinders the ventilation of drying gas in the drying process so that a long time is required for attaining sufficient dryness. Further, since pellets deformed into a flat shape have low strength, the pellets are susceptible to crushing and pulverization when fed into a reducing furnace. In addition, as the bentonite content increases, the mean grain size of green pellets decreases.
Next will be described a second disadvantage. Since bentonite remains as an impurity in reduced iron pellets, the amount of slag increases during steel-making through reduction of reduced iron pellets. This means that the product value of the reduced iron pellets is lowered. In addition, the addition of bentonite increases the cost.
According to a first aspect of the present invention, there are provided iron oxide pellets which exhibit high strength after drying and have a smaller amounts of impurities, and a method of producing the same.
According to a second aspect of the present invention, there is provided a method of producing reduced iron pellets having a high degree of metallization at high yield.
A raw material mixture according to a preferred embodiment of the present invention contains an iron oxide as the main component, a sufficient amount of a carbonaceous material for reducing the iron oxide, a sufficient amount of an organic binder for binding together the iron oxide and the carbonaceous material, and an inorganic coagulating agent in an amount of not less than 0.05 mass % and less than 1 mass %. Water is added to the raw material mixture for pelletization so as to obtain green pellets. Next, the green pellets are dried until the moisture content reduces to 1.0 mass % or less, thereby producing iron oxide pellets.
In this process, the amount of the inorganic coagulating agent contained in the raw material mixture is suppressed to 1 mass % or less, and water is added to the raw material mixture, to thereby producing green pellets. Thus, the amount of water added during pelletization can be reduced, resulting in increased strength of green pellets and minimized deformation of green pellets into a flat shape. Consequently, the passage of drying gas is not hindered, so that the pellets can be dried in a short time to a moisture content of 1.0 mass % or less. Also, the low incidence of deformation improves the strength of the resultant pellets which in turn lowers the incidence of crushing and pulverization of pellets at the time of feeding the pellets into a reducing furnace. Further, the green pellets can obtain a proper mean grain size. In addition, since the amount of the coagulating agent contained in the raw material mixture is lowered to 1 mass % or less, the coagulating agent does not remain as an impurity in reduced iron pellets, so that there is reduced the amount of slag which would otherwise be produced during the production of reduced iron.
Moreover, a dispersing agent (sodium hydroxide, etc.) having surface-activating effects may be advantageously added to the green pellets, in an amount of 0.1 mass % or less.
In this case, since the dispersing agent transforms the hydrophobic carbonaceous material into hydrophilic, moisture adequately permeates the space between the iron oxide and the carbonaceous material, resulting in improved homogeneity and strength of the iron oxide pellets.
Further advantageously, the diameter of green pellets is regulated to 6-30 mm.
In this case, stable pelletization can be performed at a constant pelletizing rate. Handling of the pellets in a reducing furnace is easy, and the diameter of the pellets does not become so large as to lower the drop test number of the pellets.
Further advantageously, the moisture content of green pellets is regulated to 11-14 mass %.
In this case, the pelletizing process becomes easy to perform, and the strength of the green pellets becomes sufficient. If the moisture content is less than 11 mass %, the pelletizing process becomes difficult. If the moisture content is in excess of 14 mass %, the green pellets become soft and flat in shape, prolonging the time required for drying.
As the oxide iron and carbonaceous material, there may be used blast furnace dust, converter dust, dust from a sintering process, electric furnace dust, or mixtures thereof.
The use of these dusts leads to reduction of the amount of industrial waste and reduction of product cost, and eliminates the need for addition of sodium hydroxide.
In the method of producing reduced iron according to a preferred embodiment of the present invention, the iron oxide pellets produced in the above-mentioned production method are fed into and reduced in a reducing furnace to thereby produce reduced iron pellets.
Since the iron oxide pellets serving as a raw material contain a smaller amount of impurities, the reduced iron pellets produced in this method contain a smaller amount of impurities, whereby high-quality reduced iron pellets having a higher degree of metallization can be produced. In addition, since the iron oxide pellets have high strength, they are difficult to crush and pulverize when fed into a reducing furnace, resulting in improvements of the yield and degree of metallization of reduced iron pellets.
Moreover, a rotary hearth furnace having a race temperature maintained at 1100-1450xc2x0 C. may be advantageously used as a reducing furnace.
In this case, since substantially no load or impact is exerted on iron oxide pellets in the reducing process by use of a rotary hearth furnace, prevention of the crushing and pulverization are prevented to a greater extent, and the yield of the reduced iron pellets is further improved accordingly.